Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, in some applications, there are advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multi-layered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
It is also highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing the relief images, in order to go from plate to press more quickly. Processes have been developed whereby photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. U.S. 2003/0180655, and U.S. 2003/0211423, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for its use in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
The photocurable layer allows for the creation of the desired image and provides a printing surface. The photocurable compositions used generally contain binders, monomers, photoinitiators, and other performance additives. Photocurable compositions useful in the practice of this invention include those described in U.S. patent application Ser. No. 10/353,446 filed Jan. 29, 2003, the teachings of which are incorporated herein by reference in their entirety. Various photopolymers such as those based on polystyrene-isoprene-styrene, polystyrene-butadiene-styrene, polyurethanes and/or thiolenes as binders may be used. Preferred binders include polystyrene-isoprene-styrene, and polystyrene-butadiene-styrene, especially block co-polymers of the foregoing.
The composition of the photocurable layer should be such that there exists a substantial difference in the melt temperature between the cured and uncured polymer. It is precisely this difference that allows the creation of an image in the photocurable layer when heated. The uncured photopolymer (i.e., the portions of the photocurable layer not contacted with actinic radiation) melts or substantially softens while the cured photopolymer remains solid and intact at the temperature chosen. The difference in melt temperature allows uncured photopolymer to be selectively removed, thereby creating an image.
The printing element is then selectively exposed to actinic radiation, which is traditionally accomplished in one of three related ways. In the first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer, which is also sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative, and the printing element is then flood exposed through the in situ negative. In the third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods produces an acceptable result, with the criteria being the ability to selectively expose the photopolymer to actinic radiation, thereby selectively curing portions of the photopolymer.
Once the photocurable layer of the printing element has been selectively exposed to actinic radiation, it may then be developed using heat. As such, the printing element is generally heated to at least about 70° C. The exact temperature will depend upon the properties of the particular photopolymer being used. However, two primary factors should be considered in determining the development temperature:                1. The development temperature is preferably set between the melt temperature of the uncured photopolymer on the low end and the melt temperature of the cured photopolymer on the upper end. This allows selective removal of the photopolymer, thereby creating the image.        2. The higher the development temperature, the quicker the process time will be. However, the development temperature should not be so high as to exceed the melt temperature of the cured photopolymer or so high that it will degrade the cured photopolymer. The temperature should be sufficient to melt or substantially soften the uncured photopolymer thereby allowing it to be removed.        
Once the heated printing element has been developed, uncured photopolymer can be melted or removed. In most instances, the heated printing element is contacted with a material that will absorb or otherwise remove the softened or melted uncured photopolymer. This removal process is generally referred to as “blotting.” Blotting is typically accomplished using a screen mesh or an absorbent fabric. Either woven or non-woven fabric is used and the fabric can be polymer-based or paper, so long as the fabric can withstand the operating temperatures involved. In most instances, blotting is accomplished using rollers to bring the material and the heated printing plate element into contact. One example of this process is described in U.S. Pat. No. 5,175,072 to Martens, the subject matter of which is herein incorporated by reference in its entirety.
Upon completion of the blotting process, the printing plate element is preferably post-exposed to further actinic radiation in the same machine, cooled, and is then ready for mounting on a printing press.
Depending upon the particular application, the printing element may also comprise other optional components. For instance, it is frequently preferable to use a removable coversheet over the photocurable layer to protect the layer during handling. If used, the coversheet is removed either just before or just after the selective exposure to actinic radiation. Other layers such as slip layer or masking layers as described in U.S. Pat. No. 5,925,500 to Yang et al. and in U.S. Pat. No. 6,238,837 to Fan, the teachings of each of which are incorporated herein by reference in their entirety, may also be used.
Equipment designed for thermally processing flexographic printing plates has traditionally been designed to accommodate plates of a particular size and shape, with larger sizes and shapes requiring substantially larger and more expensive equipment. In addition, one of the major drawbacks to many of the current thermal development systems is that these systems can only be used with flat plates, which must then be mounted after development. This requires an additional machine and more time, and can also result in a loss in accuracy when registration between multiple plates and colors is required. Thus, there remains a need in the art for a thermal developing system that is easily adaptable for processing printing elements of various sizes and shapes.
Furthermore, current thermal development systems using heated rolls for blotting away the uncured photopolymer have typically used only one heated roll that is of approximately the same width as the plate, which increases the difficulty in making printing elements of different sizes. In addition, other problems may arise when attempting to make the blotting machine of the thermal system larger to accommodate larger printing elements. A tremendous amount of force (approximately 100 pounds/linear inch) must be applied by the heated roll to force the blotting material into the image on the printing element, which can cause the heated roll to bend, resulting in an uneven floor. Also, the heating and blotting process must often be repeated several times in order to obtain effective removal of the uncured photopolymer.
U.S. Pat. No. 6,180,325 to Gelbart, the subject matter of which is herein incorporated by reference in its entirety suggests a method of applying a patterned coating to a printing element to form a mask and subsequently exposing the printing element to actinic radiation without dismounting it from the apparatus where the coating is applied. However, there is no suggestion in Gelbart that exposing and thermal development steps can be accomplished in the same apparatus.
Furthermore, exposing, developing and post exposure/detack steps have traditionally been carried out in separate devices. This requires additional time to transfer the printing element from the exposure device to the development device and can affect the quality of the finished plate as a result of handling the printing element. Thus, it would be desirable to accomplish the exposing, developing and post exposure/detack steps in the same apparatus in order to improve both the quality and the accuracy of the final product.
The basic design of the current in the round (ITR) processor is excellent for the processing cylindrical plate types, including both continuous and plate-on-sleeve types. However, if the ITR processor also had a device comparable in function to the mounting cylinder of the laser imager, it could be used for both flat and round plate types. For example, a user of cylindrical plates could continue to use the existing setup, changing mandrels as needed to ensure a proper fit between the base sleeve and the mandrel. A user wishing to process flat plates would exchange the mandrel for a mounting cylinder that would allow the attachment of flat plates to the outside of the cylinder using a vacuum, physical clamp, or some other means. The attachment of the plate can be automated, after the fashion that is already used in laser imaging devices. Moreover, the current laser imaging mounting cylinders could be used directly in an ITR-type processor.
Current laser imaging machines utilize a mounting cylinder to allow flat plate materials to be attached to the outside of the cylinder so that their masks may be laser ablated. Commercial laser imaging machines are available, for example, from Creo, Inc. and Esko-Graphics, among others. Some of these laser imaging machines include a feature by which a flat plate can automatically be drawn into the imager and held to the mounting cylinder by suitable means, such as vacuum, an edge clamp, or both vacuum and an edge clamp. It would be highly desirable to use features of this technology during thermal plate processing.
The inventors of the present invention have developed an improved thermal flexo processor that can be used to process both flat plates and CITR printing elements, and which has the further advantage of being easily scaled up or down in size by the manufacturer. Furthermore, such a device can be combined with other processes necessary for flexographic plate processing in a way that allows a single machine to combine digital ablation, exposure, processing and post-exposure steps in a single system. The net result is to create a thermal plate processor that is capable of processing both flat and round photosensitive printing elements with only minor changes, which would offer an unprecedented amount of flexibility to the system's users.